Figure 1. Variation in host dependence on symbionts and their predominant mode of transmission.

The degree to which hosts are dependent on bacterial symbionts for survival and reproduction varies hugely across taxa. Some hosts and symbionts have facultative relationships, where neither partner is dependent on each other. This may be because hosts only need symbionts in certain conditions, for example, legumes and Rhizobia, or because symbionts are only present in some of the host population, for example, aphids and Hamiltonella. In contrast, some hosts are entirely dependent on symbionts for nutrition either because they live in extreme environments, for example, deep-sea tube worms and Endoriftia, or because they have restricted diets, for example, leafhoppers and Baumannia. Image of Graphocephala coccinea sourced from phylopic.org, courtesy of Melissa Broussard available under a Creative Commons license.